The present invention relates to an ultra-directional speaker and a method for driving the ultra-directional speaker, and in particular, to an ultra-directional speaker which can provide sound with high directionality and a high sound pressure using ultrasonic waves.
This application is based on Japanese Patent Application No. Hei 9-322547, the contents of which are incorporated herein by reference.
Various ultra-directional speakers using ultrasonic waves have been proposed. Japanese Patent Application, First Publication No. Hei 3-159400 discloses an ultra-directional speaker using a sound signal and an ultrasonic signal, in which the ultrasonic signal at a particular frequency, used as a carrier wave, is amplitude modulated with the sound signal and a resulting modulated signal is output as sound from an ultrasonic vibrator.
Japanese Patent Application, First Publication No. Hei 3-296399 discloses technique in which an ultrasonic signal is amplitude modulated with a sound signal, and a resulting modulated signal is output as sound from a number of ultrasonic vibrators in the speaker which is vibrated or rotated so as to propagate the sound to a listener in a specified position.
As shown in a block diagram of FIG. 4, such speaker employs an amplitude modulator 21 for amplitude modulating the high frequency signal with the sound signal. The amplitude modulator 21 performs modulation with the audio signal from an audio generator 11 and the high frequency signal from a high frequency signal generator 51, and an electro-acoustic transducer 41 emits acoustic waves.
However, there is a problem that a high sound pressure cannot be achieved because the acoustic waves from the electro-acoustic transducer has a low amplitude. When a high sound pressure is desired with the background art, the output must be increased and an amplification factor of an amplifier 31 must be increased, and this leads to a complicated structure and increased cost.
It is therefore an object of the present invention to provide an ultra-directional speaker which can provide sound with a higher sound pressure than that of the background art without increasing cost or drastically modifying the structure, and a method for driving the ultra-directional speaker.
In order to accomplish the above object, an ultra-directional speaker of the present invention comprises: an audio generator for generating an audio signal; an ultrasonic generator for generating an ultrasonic signal; a frequency modulator for frequency modulating the ultrasonic signal with the audio signal to produce a frequency modulated ultrasonic signal; and an electro-acoustic transducer for transducing the frequency modulated ultrasonic signal into acoustic waves.
The ultra-directional speaker may further comprise an amplifier for amplifying the frequency modulated ultrasonic signal, and the amplifier may be connected between the frequency modulator and the electro-acoustic transducer. Preferably, the ultrasonic generator generates the ultrasonic signal in a frequency range of 10 kHz to 1000 kHz. A plurality of the electro-acoustic transducers may be provided, and may receive different frequency modulated ultrasonic signals, respectively. The ultra-directional speaker may further comprise a reflection plate for reflecting ultrasonic waves and audible sound waves included in the acoustic waves from the electro-acoustic transducer, and preferably the reflection plate has a curved reflection surface to allow the sound waves to converge on a particular point. The ultra-directional speaker may further comprise a filter for selectively absorbing the ultrasonic waves.
In another aspect of the present invention, a method for driving a speaker which includes an electro-acoustic transducer for outputting ultra-directional acoustic waves from an audio signal using ultrasonic carrier waves, comprises the steps of: modulating the ultrasonic waves with the audio signal to produce a frequency modulated signal; and driving the electro-acoustic transducer with the frequency modulated signal. Preferably, the ultrasonic-waves have a frequency in a range of 10 kHz to 1000 kHz. A plurality of the electro-acoustic transducers may be driven by a single frequency modulator, or by a plurality of frequency modulators which are connected to a plurality of ultrasonic generators, respectively.
The acoustic waves from the electro-acoustic transducer include ultrasonic waves and an audible audio signal. Ultra-directional sound, which has low frequency components audible to a listener, is demodulated from the acoustic waves due to nonlinear interaction during propagating via air as ultrasonic waves.
According to the present invention, a higher sound pressure than that of the background art can be achieved, and sound characteristic and sound propagation characteristic can be improved because the electro-acoustic transducer exclusively transmits the frequency modulated signal in a specified frequency band so as to emit high amplitude sound waves.